The mold for continuous casting of steel is so designed that molten steel poured from its top end is solidified by cooling and the resulting product is withdrawn from its lower end in a continuous sequence. As such, from productivity points of view, the mold is required to have a long service life. The long-life continuous casting mold heretofore known is the one disclosed in Japanese Examined Patent Publication No. 40341/1980. This mold comprises a copper or copper alloy body and as formed on the internal surface thereof which is to be exposed to molten steel, (A) an intermediate plating layer comprising at least one member selected from the group consisting of nickel and cobalt and (B) a surface alloy plating layer formed from either 3 to 20 weight % of phosphorus or 2 to 15 weight % of boron or both and the balance of at least one member selected from the group consisting of nickel and cobalt. The reasons why this mold has a long life are allegedly as follows. One of the reasons is that the provision of said intermediate layer (A) serves to flatten the gradient of hardness between the copper or copper alloy mold body which is very low in hardness and the alloy layer (B) which has a high hardness to thereby increase the bond between the three members, viz the body metal, intermediate layer and alloy layer. The other reason is that the alloy layer has high resistances to heat and wear at high temperature.
As improved versions of the above-mentioned mold, there also are known the mold carrying a chromium plating layer in superimposition on said alloy layer (B) (Japanese Examined Patent Publication No. 50734/1977) and the mold carrying an oxide layer as formed by oxidizing said alloy layer (B) (Japanese Examined Patent Publication No. 50733/1977). The chromium plating layer and the oxide layer in these molds serve to preclude deposition of molten steel splashes evolved at the start of casting on the mold surface and eliminates chances for breakout troubles. Thanks to this feature and the above-mentioned increased intimacy of the three members, namely the mold body, intermediate layer and alloy layer and the high wear resistance of the alloy layer at high temperature, these molds have serviceable lives even longer than the life of the first-mentioned mold described in Japanese Examined Patent Publication No. 40341/1980.
The above-mentioned molds carrying two or three protective layers essentially have an intermediate layer comprising at least one member of the group consisting of nickel and cobalt and, as disposed thereon, an alloy layer and, in the case of three-layer molds, further a chromium plating layer or an oxide layer, and, as such, require complicated manufacturing procedures and high production costs.